Bromocryptine prevents the decline in tuberoinfundibular neuronal release of dopamine after removal of chronic estrogen treatment

Prolonged exposure to estradiol 17-beta (E2) in rats has been shown to decrease dopamine (DA) synthesis in and release from tuberoinfundibular dopaminergic (TIDA) neurons in Fischer 344 rats. The objective of the present study was to determine whether inhibition of the E2-induced increase in anterior pituitary (AP) weight and prolactin (PRL) secretion by concomitant administration of the dopaminergic agonist, bromocryptine, could prevent the decrease in TIDA neuronal function produced by chronic E2 administration. TIDA neuronal function was evaluated by in vitro superfusion and electrical stimulation of median eminence (ME) tissue after allowing for accumulation of [3H]dopamine (DA). The effect of chronic E2 and/or bromocryptine treatment on catecholamine content in tuberohypophyseal neurons in the neurointermediate lobe was also measured to determine whether increased pituitary size possibly damaged the tuberohypophyseal neurons. Treatment with E2 for 30 days significantly increased AP weight, serum PRL concentration, and AP PRL and DNA content over values in non-E2-treated controls. When bromocryptine was injected daily during E2 treatment, bromocryptine completely inhibited the E2-induced increase in serum PRL and AP DNA content, and AP weight was only moderately increased. The evoked release of 3H at the end of the 30-day E2 treatment was reduced during electrical stimulation and there was no augmented release of 3H from the ME tissue after 10 microM nomifensine infusion in E2-treated rats and in rats given both bromocryptine and E2. However, neurointermediate lobe DA content was diminished only in E2-treated rats and not in animals given bromocryptine together with E2. When all treatments were discontinued for 30 days, animals previously given only E2 showed sustained increases in AP weight, serum PRL levels, and AP PRL and DNA content, but reduced stimulation-evoked release of 3H, absence of response to nomifensine, and reduced neurointermediate lobe DA and norepinephrine content when compared with values in non-E2-treated controls. After withdrawal of E2 treatment for 30 days, animals previously given bromocryptine and E2 together were not different from control animals in any of the parameters measured. These results suggest that the decline in TIDA neuronal release of DA induced by chronic E2 treatment was at least partly exerted via the marked hyperprolactinemia and/or by compression of the medial basal hypothalamus by the enlarged AP.     

Luteinizing hormone and prolactin are not retrograde transferred in perihypophyseal vascular complex in ewes

The objective of the study was to determine whether luteinizing hormone (LH) and prolactin (PRL) can access the brain by way of transfer from the venous blood of the cavernous sinus to the arterial blood supplying the brain and hypophysis. Studies were performed on heads of 22 mature sheep isolated during different phases of the estrous cycle and perfused with autologous blood. We were not able to demonstrate any transfer of LH and PRL in the investigated periods. This suggests that molecular weight of hormone may be a main factor determining the permeation and transfer of hormones in the perihypophyseal vascular complex.     

The influence of superovulation preparations on the levels of catecholamines in eminentia mediana, the pituitary and pineal gland of the sheep.

The influence of hormonal superovulation preparations of FSH (450 IU) or PMSG (1500 IU), on the levels of catecholamines (dopamine, norepinephrine and epinephrine) was studied in the oestrus period using radioenzymatic methods. The administration of FSH caused a significant increase in the concentrations of norepinephrine (NE) and epinephrine (EPI) in eminentia mediana (EM) of sheep (p<0.001 and p<0.01, respectively). The pituitary gland exhibited an increase in the level of norepinephrine after administration PMSG while no marked changes were recorded for epinephrine and dopamine (DA). The administration of FSH affected the increase in pituitary epinephrine (p<0.01). The hormonal stimulation by FSH resulted in a marked decrease of dopamine (p<0.05) as well as in a significant increase of norepinephrine (p<0.05) and epinephrine (p<0.05) in the epiphysis. The comparison of the effect of hormonal preparations on the changes in catecholamine levels showed that the effect of FSH was observed mostly in eminentia mediana and the pituitary gland while that of PMSG was recorded in the epiphysis.     

Blood-brain barrier permeability during dopamine-induced hypertension in fetal sheep.

Dopamine is often used as a pressor agent in sick newborn infants, but an increase in arterial blood pressure could disrupt the blood-brain barrier (BBB), especially in the preterm newborn. Using time-dated pregnant sheep, we tested the hypothesis that dopamine-induced hypertension increases fetal BBB permeability and cerebral water content. Barrier permeability was assessed in nine brain regions, including cerebral cortex, caudate, thalamus, brain stem, cerebellum, and spinal cord, by intravenous injection of the small tracer molecule [(14)C]aminoisobutyric acid at 10 min after the start of dopamine or saline infusion. We studied 23 chronically catheterized fetal sheep at 0.6 (93 days, n = 10) and 0.9 (132 days, n = 13) gestation. Intravenous infusion of dopamine increased mean arterial pressure from 38 +/- 3 to 53 +/- 5 mmHg in 93-day fetuses and from 55 +/- 5 to 77 +/- 8 mmHg in 132-day fetuses without a decrease in arterial O(2) content. These 40% increases in arterial pressure are close to the maximum hypertension reported for physiological stresses at these ages in fetal sheep. No significant increases in the brain transfer coefficient of aminoisobutyric acid were detected in any brain region in dopamine-treated fetuses compared with saline controls at 0.6 or 0.9 gestation. There was also no significant increase in cortical water content with dopamine infusion at either age. We conclude that a 40% increase in mean arterial pressure during dopamine infusion in normoxic fetal sheep does not produce substantial BBB disruption or cerebral edema even as early as 0.6 gestation.     

Activation of tuberohypophysial dopamine neurons following intracerebroventricular administration of the selective kappa opioid receptor antagonist NOR-binaltorphimine.

The effect of the kappa opioid receptor antagonist nor-binaltorphimine (NOR-BNI) was examined on the activity of dopamine (DA) neurons comprising the nigrostriatal, mesolimbic, and tuberohypophysial systems in the male rat. DA neuronal activity was estimated by measuring: (1) the concentration of the DA metabolite 3,4-dihydroxyphenylacetic acid and, (2) the accumulation of 3,4-dihydroxyphenylalanine after administration of a decarboxylase inhibitor in brain (striatum, nucleus accumbens) and pituitary regions (intermediate lobe, neural lobe) containing terminals of these neurons. The intracerebroventricular administration of NOR-BNI produced a dose- and time-related increase in the activity of tuberohypophysial DA neurons, but failed to alter the activity of nigrostriatal or mesolimbic DA neurons. The ability of NOR-BNI to enhance the activity of tuberohypophysial DA neurons was blocked by the kappa opioid agonist U-50,488. These results indicate that NOR-BNI, acting on kappa opioid receptors, activates tuberohypophysial DA neurons projecting to the neural and intermediate lobes of the pituitary.     

Rapid Release of [3H]Dopamine from Median Eminence and Striatal Synaptosomes

Release of preaccumulated, tritium-labeled dopamine ([3H]DA) from preparations of isolated nerve terminals (synaptosomes) of rat median eminence (ME) and corpus striatum (CS) was examined over short time intervals (1-20 s). In both preparations, basal efflux of [3H]DA was linear with time. Depolarization with high K+ resulted in an initial rapid release of [3H]DA which stabilized by 20 s, whereas veratridine elicited an increased rate of release over basal levels that was linear over the first 20 s. The calculated rate constants of release for both the initial phase of K+- and the veratridine-stimulated release were approximately threefold greater in CS than in ME synaptosomes. The major component of the high K+-induced release of [3H]DA from both synaptosome preparations increased as a graded function of [Ca2+]o. However, a smaller component, independent of external Ca2+, existed in both ME and CS synaptosomes. Increasing the [Mg2+] in the external solution resulted in a right shift of both the [K+]o and the [Ca2+]o dose-response curves, consistent with actions of Mg2+ on screening surface membrane charges and blocking voltage-dependent Ca2+ channels. In all studies, steady-state uptake of the [3H]DA was about twofold greater into CS than into ME synaptosomes. Moreover, the fraction of incorporated [3H]DA released by stimulation from the CS was much greater than that released from ME synaptosomes. These data are consistent with differences between these two types of dopaminergic terminals with respect to packaging and/or distribution of the accumulated neurotransmitter in intraneuronal pools, as well as marked differences in the apparent kinetics of DA release.     

Effects of acute 17alpha-methyltestosterone,acute 17beta-estradiol,and chronic 17alpha-methyltestosterone on dopamine,norepinephrine and serotonin levels in the pituitary,hypothalamus and telencephalon of rainbow trout (Oncorhynchus mykiss)

This study investigated: (a) the effects of acute 17alpha-methyltestosterone (MT) or 17beta-estradiol (E(2)) administration on norepinephrine (NE), dopamine (DA), serotonin (5-HT), 3,4, dihydroxyphenylacetic acid (DOPAC), and 5-hydroxyindoleacetic acid (5-HIAA) contents in the hypothalamus, telencephalon and pituitary of previtellogenic female rainbow trout Oncorhynchus mykiss, and (b) the effects of chronic MT administration on the levels of these neurotransmitters in these brain regions in immature male rainbow trout. The acute administration of MT induced a significant decrease in pituitary levels of DOPAC as well as in the DOPAC/DA ratio. On the other hand, the acute administration of E(2) induced an increase in pituitary 5-HT levels as well as a decrease in the 5-HIAA/5-HT ratio. In a second experiment, 20 mg MT per kilogram body weight was implanted for 10, 20 or 40 days into sexually immature male rainbow trout. Implanted rainbow trout showed increased testosterone and decreased E(2) levels. In the pituitary, MT induced long-term decreases in NE, DA, DOPAC and 5-HT levels, as well as in the DOPAC/DA ratio. Hypothalamic and telencephalic DA, NE and 5-HT levels were not affected by MT implantation. However, 5-HIAA levels and the 5-HIAA/5-HT ratio were reduced by MT implantation in both brain regions. These results show that chronic treatment with MT exerts both long-term and region-specific effects on NE, DA, and 5-HT contents and metabolism, and thus that this androgen could inhibit pituitary catecholamine and 5-HT synthesis. A possible role for testosterone in the control of pituitary dopaminergic activity and gonadotropin II release is also discussed.     

Selective Brain Cooling Reduces Water Turnover in Dehydrated Sheep

In artiodactyls, arterial blood destined for the brain can be cooled through counter-current heat exchange within the cavernous sinus via a process called selective brain cooling. We test the hypothesis that selective brain cooling, which results in lowered hypothalamic temperature, contributes to water conservation in sheep. Nine Dorper sheep, instrumented to provide measurements of carotid blood and brain temperature, were dosed with deuterium oxide (D₂O), exposed to heat for 8 days (40◦C for 6-h per day) and deprived of water for the last five days (days 3 to 8). Plasma osmolality increased and the body water fraction decreased over the five days of water deprivation, with the sheep losing 16.7% of their body mass. Following water deprivation, both the mean 24h carotid blood temperature and the mean 24h brain temperature increased, but carotid blood temperature increased more than did brain temperature resulting in increased selective brain cooling. There was considerable inter-individual variation in the degree to which individual sheep used selective brain cooling. In general, sheep spent more time using selective brain cooling, and it was of greater magnitude, when dehydrated compared to when they were euhydrated. We found a significant positive correlation between selective brain cooling magnitude and osmolality (an index of hydration state). Both the magnitude of selective brain cooling and the proportion of time that sheep spent selective brain cooling were negatively correlated with water turnover. Sheep that used selective brain cooling more frequently, and with greater magnitude, lost less water than did conspecifics using selective brain cooling less efficiently. Our results show that a 50kg sheep can save 2.6L of water per day (~60% of daily water intake) when it employs selective brain cooling for 50% of the day during heat exposure. We conclude that selective brain cooling has a water conservation function in artiodactyls.     

Persistence of low hypothalamic dopaminergic activity after removal of chronic estrogen treatment

The purpose of this study was to determine whether inhibition of tuberoinfundibular dopaminergic (TIDA) neuron function which occurs during chronic estrogen administration persists after removal of the estrogen. Ovariectomized (OVX) Fischer 344 (F344) rats were implanted for 4 weeks with a Silastic capsule containing estradiol-17 beta (E2) and controls with an empty capsule for 4 weeks. Other rats which received E2 for 4 weeks had the capsule removed and experiments performed 4 weeks later. At the end of 4 weeks of E2 treatment, anterior pituitary (AP) weight was increased sixfold, serum prolactin (PRL) 65-fold, and AP DNA content fivefold over OVX control rats. Four weeks after removal of E2, AP weight, serum PRL, and AP DNA content declined, but remained significantly above OVX control values. At the end of 4 weeks of E2 treatment and after E2 withdrawal, release of [3H]dopamine (DA) from median eminence (ME) tissue superfused in vitro was lower than from ME of OVX control rats although [3H]DA accumulation was not significantly different among the treatment groups. Administration of apomorphine (APO), a dopamine agonist, significantly reduced plasma prolactin levels in OVX control rats, in rats at the end of 4 weeks E2 treatment, and in rats after 4 weeks of E2 withdrawal. Injection of haloperidol (HALO) produced similar increases in plasma PRL/estimated PRL-cell DNA in OVX controls, at the end of E2 treatment or after E2 withdrawal. However, injection of morphine (MOR), a drug which increases the release of PRL by inhibiting hypothalamic dopaminergic activity, resulted in a rise in plasma PRL/estimated PRL-cell DNA in OVX control rats that was significantly greater compared to rats at the end of E2 treatment or after E2 withdrawal. Since rats treated with E2 released less [3H]DA from ME tissue in vitro, and were less responsive to MOR, it can be that animals treated for 4 weeks with E2 show a decreased ability to release DA from TIDA neurons which persists even after termination of E2 treatment. These results suggest that chronic high circulating E2 levels result in a depression of TIDA neuronal activity which is sustained after E2 is removed.     

Hypophyseal angioarchitecture of common tree shrew (Tupaia glis) revealed by scanning electron microscopy study of vascular corrosion casts.

The vascular corrosion cast technique in conjunction with scanning electron microscopy (SEM) was used for the study of pituitary microvascularization in the common tree shrew (Tupaia glis). The pituitary vascular casts were obtained by infusion of low viscosity methyl methacrylate plastic (Batson's no.17) mixture. It was found that the blood supplies to the pituitary complex were from branches of the circle of Willis and could be divided into two groups. The first group consisted of two to four superior hypophyseal arteries (SHAs) branching off from the internal carotid artery supplying each half of the median eminence (ME), infundibular stalk (IS), and pars distalis (PD). The SHAs supplying the ME branched into internal and external capillary plexi. The internal plexus had a larger capillary size (approximately 15 microns in diameter), was deeper in position, and had denser and more complex capillary loops than those in the external plexus. The capillaries of the external plexus were approximately 10 microns in diameter. The two plexi drained into 15-20 hypophyseal portal veins (HPVs) which were located mainly along the ventral and ventrolateral surfaces of the IS before breaking up into large capillaries (approximately 18 microns in diameter) with an anteroposterior arrangement within the PD. The second group consisted of one inferior hypophyseal artery (IHA) on each side branching off from the internal carotid artery. These arteries gave off branches to pierce the dorsolateral and ventrolateral aspects of infundibular process (IP) before branching off to form a capillary network. They also gave rise to radiating capillaries to supply the pars intermedia (PI) surrounding the cortical area of the IP. The hypophyseal cleft separating the PI from the PD was clearly seen with very few blood vessels. The capillaries in both PD and IP joined to form confluent hypophyseal veins draining the blood into the cavernous sinus.     

The effect of chronic chlorpromazine administration on monoamine levels in various regions of rat brain

The neuroleptic drug, chlorpromazine (CPZ) has been shown to exert its antipsychotic effect by blocking post synaptic dopamine receptors. However, its effect on steady state levels of monoamines is still in discrepancy. In the present study, CPZ (4 mg/kg body weight) was administered intraperitoneally to adult Wistar rats chronically for 75 days and the levels of norepinephrine (NE), dopamine (DA) and 5-hydroxytryptamine (5-HT) were assayed in various brain regions by high performance liquid chromatography (HPLC). After the experimental period body and brain weights were not statistically different from controls. NE and 5-HT levels were increased only in hippocampus by 15% (p<0.01) and 16% (p<0.01) respectively. DA levels were consistently increased in cortex by 39% (p<0.001), striatum-accumbens by 18% (p<0.01), hippocampus by 27% (p<0.01), hypothalamus by 34% (p<0.001), cerebellum by 36% (p<0.001) and brainstem by 40% (p<0.001) in CPZ treated rats compared to controls. The results suggest that chronic CPZ administration increases DA levels in almost all regions of brain and reflect the ability of CPZ to preferentially interfere with synaptic transmission mediated by DA in brain. It also suggests that this increase in DA might be responsible for certain side effects seen in patients after chronic CPZ treatment.     

Tamoxifen counteracts estradiol induced effects on striatal and hypophyseal dopamine receptors

We investigated the ability of Tamoxifen (TAM), an antiestrogen drug, to counteract the modifications induced by estrogens on dopamine (DA) receptors on striatum and on adenohypophysis of ovex female rats. Subacute treatment with 17 beta-estradiol (E2) at both low (0.1 micrograms/kg) and high (20 micrograms/kg) doses confirmed its ability to increase the number of striatal 3H-Spiperone (3H-SPI) binding sites in a dose dependent manner. By contrast in the pituitary, only high doses of estrogen were effective in reducing the number of DA receptors. We treated ovex female rats for 15 days with TAM alone or associated with E2, to see if these estrogenic effects could be suppressed by an antiestrogenic drug. TAM did not affect the number of striatal DA receptors, but significantly increased the adenohypophyseal DA binding sites, without varying their affinity. No changes were observed in pituitary and striatal DA receptor density, even when TAM was injected in association with estradiol. In conclusion: TAM is able to counteract the effects estrogens have on DA receptors. However there is some evidence that it could influence the pituitary DA systems independently of its antiestrogenic activity.     

Prostaglandin E2 stimulates adrenocorticotrophin and cortisol secretion via a hypothalamic site of action in fetal sheep.

The hypothesis that prostaglandins stimulate fetal adrenocortical activity via a central site of action within the fetal brain was tested in chronically catheterized fetal sheep. At day 120 gestation (term = 145 days) fetal sheep were surgically prepared with catheters in the lateral cerebral ventricle, jugular vein and carotid artery and experiments began five days later. Intravenous (i.v.) infusion of prostaglandin E2 (30 or 120 micrograms.h-1) caused a significant dose-related increase in fetal plasma concentrations of ACTH. Despite this increase in ACTH, cortisol was only stimulated after the highest dose of prostaglandin E2. Intracerebroventricular (i.c.v.) infusion of PGE2 (30 micrograms.h-1) also stimulated ACTH secretion although the peak response was delayed and considerably less compared with the same dose administered intravenously. Prostaglandin F2 alpha administered i.v. or i.c.v. had no effect on circulating concentrations of either ACTH or cortisol. These data provide evidence that prostaglandin E2 can stimulate fetal ACTH secretion by acting in the fetal brain. Furthermore, the greater release of ACTH after i.v. compared with i.c.v. prostaglandin E2 suggests that a site of action other than the brain, such as the pituitary gland, may also be important. These results provide further evidence that during late gestation circulating prostaglandins can act to stimulate fetal pituitary-adrenal maturation.     

Dopaminergic modulation of some central actions of angiotensin II in vivo

Studies were performed in 12 conscious sheep of both sexes to determine if a brain dopaminergic pathway is involved in modulating the central actions of angiotensin II (Ang II) in regulating body temperature and plasma renin activity (PRA). Previous data showed that intracerebroventricular (ICV) infusion of Ang II significantly decreased PRA and body temperature. In contrast, converting enzyme inhibitor SQ 20881 (SQ) or dopamine (DA) significantly increased PRA and body temperature of sheep. In the present study, ICV infusion of the DA antagonist metoclopramide (MCP) (20 micrograms/min) significantly decreased PRA to 68 +/- 5% of the basal level. When sheep were pretreated with ICV MCP (20 micrograms/min) for 2 hr and then infused ICV with MCP (20 micrograms/min) plus DA (20 micrograms/min), Ang II (25 ng/min), or SQ (1 microgram/min), the PRA and temperature responses to DA, Ang II, or SQ were all abolished or attenuated significantly. The converse did not hold. Sheep pretreated with SQ (1 microgram/min) still showed a significant increase in body temperature (0.43 +/- 0.05 degree C) when infused with DA (20 micrograms/min). These results support the hypothesis that a central DA pathway is involved in the modulation of the actions of centrally administered Ang II on temperature and PRA.     

Substance P and cholecystokinin regulate neurochemical responses to cocaine and methamphetamine in the striatum

The mechanism of action of drugs of abuse like cocaine and amphetamines has been studied extensively in the dopamine terminal field areas of the caudate-putamen (CPu) and the nucleus accumbens (NAc) of the rodent brain. These brain regions contain several neuropeptides that must play important roles in the normal physiological functions of these brain regions. The study of neuropeptide physiology in the context of the neurobiological responses to drugs of abuse may shed some light on the intrinsic mechanism of action of neuropeptides of the CPu and the NAc. The neuropeptides substance P (SP) and cholecystokinin (CCK) are present in the striatum where they could play an important role regulating the effects of psychostimulants like cocaine and amphetamines (methamphetamine [METH] is a long acting derivative of d-amphetamine). These highly addictive agents induce the release of dopamine (DA) (and other catecholamines) from dopaminergic terminals of the striatum. The excessive release of DA in the striatum and the NAc has been implicated in the habit-forming properties of these drugs. In order to study the contribution of SP and CCK in the striatum during psychostimulant treatment, we employed selective non-peptide neurokinin-1 (NK-1) and cholecystokinin-2 (CCK-2) receptor antagonists that readily cross the blood brain barrier. We infused the neurokinin-1 receptor (NK-1R) antagonist, L-733,060, into the striatum of freely moving rats via a microdialysis probe in order to assess the effects of SP on cocaine-induced DA overflow in the striatum. Infusion of the NK-1R antagonist prior to a systemic injection of cocaine (10 mg/kg i.p.) significantly attenuated DA overflow in the striatum. Conversely, infusion of a CCK-2 receptor (CCK-2R) antagonist, L-369,293, through the microdialysis probe evoked DA overflow in the striatum in the absence of cocaine and potentiated DA overflow after a single injection of cocaine (10 mg/kg i.p.). Exposure to METH (10 mg/kg 4x at two-hour intervals) produced deficits of dopamine transporters (DAT) in mice striatum that are detectable three days after the treatment and are long lasting. Pre-treatment (i.p. injections) with the NK-1R antagonist, WIN-51,708 30 minutes before the 1st and 4th injections of METH prevented the loss of DAT in the striatum. Moreover, pre-treatment with the NK-1R antagonist prevents METH-induced cell death. Taken together, these results demonstrate that the NK-1R and the CCK-2R are important modulators of the actions of the psychostimulants cocaine and METH. Neuropeptide receptors represent an important control point mediating the effects of the neurotransmitter DA in the striatum of the rodent brain.     

Effects of intracranial self-stimulation on brain opioid peptides

The neurochemical system(s) underlying brain stimulation reward (ICSS) has been investigated for many years. The catecholamine hypothesis is currently most accepted with predominant emphasis on the role of dopamine. The present report examines the role of three opioid peptides--Methionine and Leucine Enkephalin (ME and LE) and beta-Endorphin (beta-E) in this behavior. Peptide levels from pituitary, hypothalamus and whole brain were determined by independent RIAs and analyzed according to treatment: low, moderate and high ICSS responders, sham controls, animals receiving nonspecific stimulation, and naloxone--with and without ICSS. Not only did naloxone reduce ICSS from high responders by 74%, it also was able to reduce peptide levels--most notably for ME and beta E in most regions. Additionally, the effects of ICSS on endorphin levels was found to be related to the rate category of responding. Since endorphins are known to interact with dopamine systems, it is therefore considered likely that the endogenous opioid peptides play an important role in ICSS either directly or indirectly via their influence on catecholamine systems.     

Depletion of brainstem epinephrine stores by alpha-methyldopa: possible relation to attenuated sympathetic outflow

The antihypertensive effect of alpha-methyldopa (MD) is believed to be critically dependent on its ability to deplete endogenous catecholamines or cause the synthesis of false neurotransmitters. We used liquid chromatography with electrochemical detection (LCEC) and negative chemical ionization gas chromatography-mass spectrometry (GC-MS) for quantitation of catecholamines and MD metabolites in rat. MD intraperitoneally (100 mg/kg q12 hr X 12 days), significantly increased alpha-methylnorepinephrine (MNE) in brain (1.02 +/- 0.33 micrograms/g), heart (1.67 +/- 0.57 micrograms/g) and adrenal glands (114.93 +/- 50.47 micrograms/g) Endogenous norepinephrine (NE), epinephrine (E) and dopamine (DA) were reduced. ME levels were 2.19 +/- 0.44 micrograms/g (n = 6) in the adrenal gland but only 99 +/- 26 pg/g (n = 3) in the brainstem. MD-induced endogenous brainstem NE depletion was more than compensated by MNE production, but brainstem E depletion was not compensated for by a stoichiometric production of brainstem ME. We conclude (1) although ME is a metabolite of MD, it is present in extremely low concentrations in brainstem and (2) central epinephrine-containing neurons are depleted of neurotransmitter by MD therapy. If this selective epinephrine depletion occurs in the bulbospinal tract neurons responsible for maintaining sympathetic tone, then this effect could contribute to the antihypertensive effect of MD.     

Role of 5-hydroxytryptamine in the regulation of brain neuropeptides in normal and diabetic rat

The effect of 5-hydroxytryptamine (5-HT) alteration on brain dopamine (DA), norepinephrine (NE), beta-endorphin (beta E) and immunoreactive insulin (IRI) was studied in Sprague-Dawley diabetic and control rats. Diabetes was induced using alloxan (45 mg/kg), 15 days prior to sacrificing. Both control and diabetic animals were treated with either p-chlorophenylalanine (PCPA, 300 mg/kg) 3 days prior to sacrificing or fluoxetine (10 mg/kg) twice daily for 3 days. PCPA treatment significantly decreased brain content of 5-HT and 5-hydroxyindole acetic acid (5-HIAA) while it caused significant increase and decrease in brain beta E and insulin levels, respectively, in both normal and diabetic rat. Meanwhile, the administration of fluoxetine resulted in significant increase in brain content of 5-HT, DA, NE and insulin but significant decline of beta E in diabetic and saline control rats. The results of this experiment indicate that 5-HT may be regulating both beta E and insulin regardless of the availability of pancreatic insulin.     

Chronic estradiol exposure induces oxidative stress in the hypothalamus to decrease hypothalamic dopamine and cause hyperprolactinemia

Estrogens are known to cause hyperprolactinemia, most probably by acting on the tuberoinfundibular dopaminergic (TIDA) system of the hypothalamus. Dopamine (DA) produced by TIDA neurons directly inhibits prolactin secretion and, therefore, to stimulate prolactin secretion, estrogens inhibit TIDA neurons to decrease DA production. However, the mechanism by which estrogen produces this effect is not clear. In the present study, we used a paradigm involving chronic exposure to low levels of estradiol-17β (E(2)) to mimic prolonged exposures to environmental and endogenous estrogens. We hypothesized that chronic exposure to low levels of E(2) induces oxidative stress in the arcuate nucleus (AN) of the hypothalamus that contains TIDA neurons and causes nitration of tyrosine hydroxylase (TH), the rate-limiting enzyme in the synthesis of DA. This results in a significant decrease in DA and consequently, hyperprolactinemia. To investigate this, adult, intact female cycling rats were implanted with slow-release E(2) pellets (20 ng/day) for 30, 60, or 90 days and were compared with old (16-18 mo old) constant estrous (OCE) rats. Chronic E(2) exposure significantly increased the expression of glial fibrillary acidic protein and the concentrations of interleukin-1β (IL-1β) and nitrate in the AN that contains perikarya of TIDA neurons and increased nitration of TH in the median eminence (ME) that contains the terminals. These levels were comparable to those seen in OCE rats. We observed a significant decrease in DA concentrations in the ME and hyperprolactinemia in an exposure-dependent manner similar to that seen in OCE rats. It was concluded that chronic exposure to low levels of E(2) evokes oxidative stress in the AN to inhibit TIDA neuronal function, most probably leading to hyperprolactinemia.     

A short-term diabetes induced changes of catecholamines and p38-MAPK in discrete areas of rat brain

Chronic diabetes is associated with the alteration of second messengers and CNS disorders. We have recently identified that protein kinases (CaMKII and PKC-alpha) and brain neurotransmitters are altered during diabetes as well as in hyperglycemic and acidotic conditions. In this study, we investigated the effects of acute diabetes on the levels of dopamine (DA), norepinephrine (NE), epinephrine (E) and p38-Mitogen-Activated Protein Kinase (p38-MAPK) in striatum (ST), hippocampus (HC), hypothalamus (HT), midbrain (MB), pons medulla (PM), cerebellum (CB) and cerebral cortex (CCX). Alloxan (45 mg/kg) diabetic untreated rats that showed hyperglycemia (>260 mg%), revealed significant increases of DA level in ST (1.5 fold), HC (2.2 fold) and PM (2.0 fold) and the E level also found to be increased significantly in HT (2.4 fold), whereas the NE level was decreased in CB (0.5 fold), after 7 days of diabetes. Immunoblotting study of p38-MAPK expression under identical conditions showed significant alterations in ST, HC, HT and PM (p<0.05) correlated with the changes of catecholamines (DA and E). On the other hand, the above changes were reversed in insulin-treated diabetic rats maintained under normal glucose level (80 -110 mg %). These results suggest that p38-MAPK may regulate the rate of either the synthesis or release of DA and E in corresponding brain areas, but not NE, under these conditions.